from openpyxl import Workbook
from openpyxl.cell.text import InlineFont
from openpyxl.cell.rich_text import TextBlock, CellRichText


rich_string1 = CellRichText(
    'This is a test ',
    TextBlock(InlineFont(b=True), 'xxx'),
   'yyy'
 )

wb = Workbook()
ws = wb.active
ws['A1'] = rich_string1
ws['A2'] = 'Simple string'
wb.save('rich.xlsx')